In order to optimize efficiency of a power amplifier one needs to keep a supply voltage of the power amplifier as low as possible; in this document the efficiency means 1—power losses/output power. On the other hand the supply voltage has to be high enough that no clipping occurs in an output signal of the power amplifier. Keeping the supply voltage at an optimal value is a challenging task especially when an impedance of a circuitry loading the power amplifier varies. For example, in a mobile communication device an impedance of an antenna loading a power amplifier varies considerably with frequency and with external circumstances. An example of external circumstances that has an effect on the impedance of the antenna is the position of user's fingers in the vicinity of the antenna, i.e. a ‘finger effect’. In a mobile communication device the impedance of an antenna can vary over a wide range, characterized by a voltage standing wave ratio (VSWR) reaching up to 10:1. Variations in the impedance can be taken into account by keeping a value of the supply voltage so high that there is a sufficient safety margin in different situations. This kind of approach leads, however, to a situation in which the supply voltage is unnecessarily high over a significant portion of time. Too high a supply voltage means unnecessary power losses especially in output stage transistors of a power amplifier.